One of the development trends of radio frequency (RF) communication systems is to simplify circuit designs in order to minimize a chip size as well as to reduce hardware costs. A direct conversion receiver, although having a structure much simpler than that of a superheterodyne receiver, is not prevalent due to being limited by hardware techniques. Accompanied with advancement in manufacturing and circuit techniques, a direct conversion receiver with good performance is gradually becoming possible and is thus increasingly valued in the communication field.
FIG. 1 shows a partial structural view of a typical conventional direct conversion receiver. A preliminary filter 11 first removes noise outside a target frequency band from an RF signal, which is then amplified by a low-noise amplifier 12 providing a predetermined gain. The RF signal is then demodulated by mixers 13 and 14 to generate an in-phase baseband signal I and a quadrature baseband signal Q. However, the direct conversion receiver suffers from a drawback that DC offset, flicker noise and second-order inter-modulation distortion (IMD2) caused by the mixers 13 and 14 due to the RF signal being converted to a baseband signal through one-time mixing, all enter into the frequency band of the baseband signal, such that a signal-to-noise ratio of the signal is lowered.
FIG. 2 shows a relationship diagram between input and output efficiencies of a first-order signal and a second-order signal in the mixer 13. An upper curve represents a relationship between the input and output powers of the first-order signal and a lower curve represents a relationship between the input and output powers of the second-order signal. A power corresponding to an intersecting point of two virtually extended lines of the two curves is commonly referred to as a second-order input intercept point (IIP2). As a value of the second-order input intercept point gets greater, the power of the second-order signal outputted by the mixer gets smaller, which is considered as a more ideal situation.
FIG. 3 shows a schematic diagram of an example of input/output signal spectrum of the mixer 13. Before entering the mixer 13, a target signal carrying valid data, indicated by lined blocks, is distributed at two sides of a central frequency fLO. As shown in FIG. 3, near the center frequency fLO are two interference signals respectively occurring at positions of frequencies fLO+f1 and fLO+f2. When a difference between the frequencies f1 and f2 is insubstantial, a second-order inter-modulation distortion, caused by the two interference signals after passing through the mixer 13 that has a second-order non-linear characteristic, falls near the direct-current band to impose significant undesirable effects on the data signal. The situation is further worsened by the incapability of removing the second-order inter-modulation distortion with a subsequent baseband processing procedure. The same issue also takes place in the mixer 14.
Many factors may constitute the second-order inter-modulation distortion. Apart from the abovementioned external noise interferences, a mismatch between the secondary harmonic of the RF signal and components of the mixers 13 and 14 also accounts for a reason of the second-order inter-modulation distortion. As the second-order input intercept point gets greater, the second-order inter-modulation distortion energy in FIG. 3 gets lower. Therefore, circuit standards of many direct conversion receivers have a strict standard for a minimum value of the second-order input intercept point. Further, characteristics of the second-order input intercept point are also regarded as important indices for estimating performance of a direct conversion receiver. Therefore, there is a need for a solution for reducing the second-order inter-modulation distortion in a direct conversion receiver to correspondingly increase the second-order input intercept point.
A currently prevalent Wideband Code Division Multiple Access (WCDMA) communication apparatus is particularly strict on requirements of the second-order input intercept point. In a WCMDA system, a transmitting circuit and a receiving circuit not only are located near each other but also operate concurrently. Supposing the characteristics of the second-order input intercept point of the receiving circuit are unsatisfactory, interference caused by data transmission of the transmitting circuit are then likely to hinder the receiving circuit from successfully receiving external data.